ABSTRACT
Modern fluvial deposits can be affected by early diagenetic element mobilization at redox boundaries between oxygenated and oxygen‐free zones near the groundwater level. The visible product ...of redox transformation of Fe is the sediment colour. Distinct red and black sediment layers have been studied in eight shallow (<4.2 m depth) sections in the floodplain of Morava River, Czechia. The aim was to investigate the composition, origin, rates of formation and stratigraphic significance of the red strata using bulk‐rock analytical methods, radiocarbon dating, optical and scanning electron microscopy coupled with in situ (energy‐dispersive X‐ray scanning electron microscopy and laser ablation inductively coupled plasma mass spectrometry) geochemistry. The coloured layers, in places well‐cemented, developed in permeable sands and gravels above the water table, close to the boundary with overlying less permeable floodplain sandy silts. Their colour is due to Fe and Mn oxyhydroxides (goethite, haematite, todorokite and birnessite) coatings of framework grains. Black, Mn‐rich layers occur stratigraphically higher than the red, Fe‐rich ones. The coatings are a few‐hundred microns thick at maximum, often botryoidal, composed of alternating Fe‐rich and Mn‐rich laminae enriched in As, Mo, Sb, P, Cu and U. The coatings formed under suboxic conditions due to Fe and Mn oxyhydroxide cycling, driven partly by microbial activity, at the groundwater‐related redox boundary which resulted in the distinct vertical arrangement of the black and red layers. Being markedly enriched in As bound to Fe‐oxyhydroxides and exposed to fluctuating redox conditions, the coloured strata can potentially release As to groundwater and represent serious environmental hazards. Radiocarbon ages suggest that the groundwater‐type red beds can occur extremely quickly, over a few hundred years. It is assumed that, under suitable circumstances, this reddening can be preserved in the rock record, and represent a specific model for the development of continental red beds.
Microclimatic processes were studied in the soil of the well-defined “breathing spot” named Půlhodina (BSP) in the Hranice Karst (Czech Republic). The results have indicated that air is exchanged ...between the external atmosphere and a supposed underground cavity under air-buoyancy control. Upward airflow ventilation mode (UAF mode) and downward airflow ventilation mode (DAF mode) were distinguished. The virtual temperature of switching between both modes was about 16.5 °C. At the UAF mode, the underground air is rising from the cavity through the BSP, warms soil, and carries out the underground CO
2
. At the DAF mode, the external air is sucked into the cavity through the BSP soil, promoting the advective flux of the soil-respired CO
2
deeper into the cavity. Whereas the warming of the soil is a dominant process in the UAF mode, the CO
2
advective flux into the cavity is the main process in the DAF mode. The simplified mathematical model was derived as a function of the external air temperature and the soil air temperature to simulate the time evolution of CO
2
concentration in the BSP. Despite simplifying assumptions, this model showed a good agreement with the collected data. The advective CO
2
flux from a breathing spot soil may significantly contribute to the total CO
2
flux into the adjacent underground cavity. Its total values may reach 4 × 10
−3
mol s
−1
or even more, based on the actual CO
2
concentration, the exchanged airflow magnitude (both controlled by external temperature), and the breathing spot area. This conclusion is important for karsologists studying the cave CO
2
budget, CO
2
sources, and the mechanisms of CO
2
transport into caves.
Mercury (Hg) chemostratigraphy is an efficient tool for tracing distal volcanism, as suggested in many recent studies. In our research, we focused on Hg and other trace element contents in two ...Devonian-Carboniferous boundary sections from two different and spatially distant areas. The Lesní lom section (Czech Republic) was located in the south-eastern part of the Laurussia plate and the Duli (Guangxi, China) section in the South China Plate. In both studied sections, the highest Hg, Hg/Fexs or Hg/Al values occurred at the Devonian-Carboniferous boundary. In Lesní lom, organic matter remineralization was connected with origin of pyrite which sequestered a part of the Hg content. The decoupling of Hg and TOC within the Duli section strata and the significant correlation with Al and Fe may suggest that Hg is hosted primarily by clay particles, derived from the landmass into the marine realm. Mercury was also incorporated into pyrites but the poor correlation between Hg and TS (total sulfur) indicates a possible loss of S during oxygenation events. In the Lesní lom section, and partly in the Duli section, Fe and Mn redox cycling may have played an important role in sediment Hg content. Both sections were influenced by widespread hydrothermal and volcanic activity, indicated by positive Eu anomalies, MSI, Fe/Ti, Zr/Al2O3 and Ti/Al2O3 values. It remains to be clarified whether, besides the interplay of local magmatic activity and redox oscillations, there may have been some additional influence of the multi-phase Viluy Large Igneous Province, although its younger phase (364.4 ± 1.7 Ma) slightly precedes the Hg anomalies obtained.
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•Distribution of Hg and other trace elements from Devonian-Carboniferous boundary•The highest Hg, Hg/TOC, Hg/Al, and Hg/Fe values bellow and in the Hangenberg Events•Anomalous Hg was not simply derived from the increased deposition of organic carbon.•Both sections were influenced by widespread hydrothermal and volcanic activity.•Hg enrichment caused by local hydrothermal and volcanic activity and redox variations
Defining anomalies from geochemical background is a principal step in anthropogenic contamination studies and environmental risk assessment, and yet the complexity of processes affecting the ...composition of sediments makes this task extremely difficult. We address grain size and sediment provenance as two parameters influencing the geochemistry of contaminated fluvial sediments. We analysed 576 sediment samples from channels, floodplains, oxbow lakes, and dam reservoirs of the Morava River, Czech Republic. The geochemical data are compared with grain size and statistically processed using primary observations and log-ratios. An effective way to remove the grain size and provenance effect is to use enrichment factors calculated from least trimmed squares regression of Pb, Zn, Cu and Cr data vs. normalizing Ti, which is sensitive to grain size. Based on empirical testing, titanium proved more suitable for normalizing than did Al and Rb. Isometric log-ratio coordinates of the Pb, Zn, Cu and Cr data show a strong misfit with those defined by the enrichment factors. A log-ratio approach considering just subcompositions of elements with similar geochemical behaviour provides much better results.
•Toxic metal concentrations are strongly influenced by grain size in fluvial sediments.•Ti is a better normalizing element than Al and Rb, but empirical testing is needed.•Robust LTS regression of Pb, Zn, Cu, Cr vs. Ti removes the grain-size effect.•Suitable subcompositions are needed for log-ratio approach to background definition.
We present a comprehensive study concerning sedimentary processes in fluvial sediment traps within the Morava River catchment area (Czech Republic) involving three dammed reservoirs, four meanders ...and oxbow lakes, and several natural floodplain sites. The objective of the study was to determine sediment accumulation rates (SAR), estimate erosion rates, calculating these using a combination of the 137Cs method and historical data. Another purpose of this study was to provide insight into changing erosion and accumulation rates over the last century. Extensive water course modifications were carried out in the Morava River catchment area during the twentieth century, which likely affected sedimentation rates along the river course. Other multiproxy stratigraphic methods (X-ray densitometry, magnetic susceptibility, and visible-light reflectance spectrometry) were applied to obtain additional information about sediment infill. Sediment stratigraphy revealed distinct distal-to-proximal patterns, especially in reservoirs. Granulometrically, silts and sandy silts prevailed in sediments. Oxbow lakes and meanders contained larger amounts of clay and organic matter, which is the main difference between them and reservoirs. Pronounced 137Cs peaks were recorded in all studied cores (maximum 377Bq·kg−1), thus indicating Chernobyl fallout from 1986 or older events. Calculated sediment accumulation rates were lowest in distal parts of reservoirs (0.13–0.58cm/y) and floodplains (0.45–0.88cm/y), moderately high rates were found in proximal parts of reservoirs and oxbow lakes (2.27–4.4cm/y), and the highest rates in some oxbow lakes located near the river (6–8cm/y). The frequency of the inundation still can be high in some natural areas as in the Litovelské Pomoraví protected area, whereas the decreasing frequency of the inundation in other modified parts can contribute to a lower sedimentation rate. The local effects such as difference between SARs in oxbow lakes and reservoirs, different grain size distribution in both systems, and high variability in thickness of their proximal and distal parts play a crucial role in the analysis of regional accumulation rates. Local effects are much stronger than regional effects, such as rainfall and land use. Combined with the low resolution of time scales (usually only three datums are available: reservoir construction datum, 137Cs fallout event, and top of sediment), these effects may obscure the general trends of regionally increasing or decreasing net SARs, making the analysis of erosion rates from the sedimentary record an extremely difficult task.
•Studied reservoirs and oxbow lakes can have high trap efficiency.•The reservoirs displayed more heterogeneous grain-size and sediment-thickness patterns than did the abandoned meanders.•The lowest accumulation rates were found in distal parts and the highest in proximal parts of reservoirs and oxbow lakes.•The main difference is in the continuity of the sedimentary record, which is more complete in reservoirs.
Loess-palaeosol sequences represent the most complete terrestrial archives of Quaternary climatic cyclicality. Particle size and geochemistry are widely used proxy data in palaeoclimatologic analysis ...of loess-palaeosol sequences. The palaeoclimatologic signals hidden in the texture and chemical composition of the Czech loess-palaeosol sequences, which are part of the European loess belt, are modulated by temporal changes in the interplay of oceanic and continental macroclimates and by the diverse bedrock geology of the Bohemian Massif.
Innovative tools of compositional data analysis, including log-ratio transformation and scalar-on-function regression, can substantially enhance the information value of large granulometric and geochemical datasets, when compared to classical statistics of raw data. Particle size distribution and bulk-rock geochemistry of 389 and 542 samples, respectively, from four Czech loess-palaeosol sequence sections representing the last glacial-interglacial cycle were analysed in this study. Centered log-ratio transformation was applied to the key elemental proxies and their spatial (between sections) and stratigraphic (within section) distribution. Centered log-ratio transformed densities of key elements were then plotted against particle size distribution to assess the particle size control on element concentrations.
Nearly all loess-palaeosol sequence samples exhibit a bimodal particle size distribution with medium/coarse silt and coarse clay fraction representing the main modes. The dominant silt fraction is completely allogenic. The clay component is partly allogenic, transported by high altitudes air stream, and partly authigenic (neoformed in situ by pedogenic processes). However there is only a minor variation in particle size between the sections, the centered log-ratio transformed density functions reveal that the particle size control on the distribution of major and trace elements is highly site-specific. The provenance signal is recorded especially in coarser-grained fractions transported for a short distance from the source area by near-surface wind. The proportion of the authigenic clay fraction, the alteration of feldspars, micas and low alkali contents indicated by high Rb/K ratio and high values of Rb/Sr and Sr/Ca ratios, which indicate the intensity of carbonate leaching, reflect the intensity of chemical weathering, which is highest in the podzols. Precipitation rates were likely the main microclimatic factor that controlled the compositional differences. The loess-palaeosol sequences in areas with higher present-day annual precipitation show higher contents of clay minerals, higher degree of loessification (cementation) and subsequent pedogenic changes.
•The studied loess and soils reveal bimodal grain-size distribution.•The mineral and chemical composition of loess is controlled by grain size.•Scalar-on-function regression reveals local influences on the LPS formation.•Local precipitation rates affect compositional characteristics of loess and soils.
The processes responsible for reddening of Continental Red Beds (CRBs) and the relationship between color variation and paleoenvironmental conditions are presented focusing on a comprehensive ...multi-proxy study of Permian sediments in the Bohemian Massif, Czechia. The investigation incorporates facies analysis, quantitative color assessment using diffuse Vis-spectral reflectance (DRS), optical and electron microprobe microscopy, bulk-rock (XRF and XRD), and in-situ geochemistry (laser-ablation ICP-MS). Results indicate a progressive drying trend from the Cisuralian to Guadalupian series in studied continental red sediments. Different facies indicate the change of the sedimentary environment from a deep lacustrine environment (lower part of Rudník Member, Cisuralian) to a fluvial floodplain and eolian environment (Trutnov Formation, Guadalupian). Examination of the three major categories (white, gray–green and red sediments) identified in the studied continental red beds indicates that diagenetic alteration of clay minerals and biotite was the main source of iron fueling the growth of hematite responsible for their red color. Early diagenetic processes and paleoenvironmental conditions, particularly the oxidizing or reducing conditions play a key role in the red sediment formation. It is suggested that later diagenetic stages are incapable of coloring non-red, iron-rich sediments formed in deep anoxic lacustrine environments. Microbial activities and reducing fluids have been identified as the main factors in the formation of gray–green sediments forming distinct reduction zones. The reduction spots formed during the early stages of diagenesis (eodiagenesis), and they were likely never red. In contrast, reduction strips, initially exhibiting a red hue, underwent a color change during more advanced stages of diagenesis (mesodiagenesis).
This study is focused on the lithology and provenance of late Quaternary fluvial deposits of the Upper Morava Basin a pull-apart basin situated at the contact of the Bohemian Massif and Western ...Carpathians. Late Cenozoic tectonic convergence between these two units caused differential subsidence along strike-slip faults of the Elbe-Odra zone, leading to a distinct horst-and-graben morphology of the Upper Morava Basin. The Pleistocene fluvial deposits are preserved in several terrace levels and partly buried under the present-day floodplain of the Morava River. This study is based on four cores (11–25m deep) drilled in the floodplain of two major depocentres of the basin, the Lutín Graben, and the Upper Morava Basin sensu stricto. The drill cores were analysed for grain size, pebble- and heavy-mineral composition, chemical composition of detrital garnets, bulk magnetic susceptibility, sediment colour (visible-light spectral reflectance) and bulk element geochemistry. Age interpretations are based on eight optically stimulated luminescence (OSL) analyses. The Upper Pleistocene sediments were deposited in a gravelly braided to transitional braided-meandering river in both the Upper Morava Basin s.s. and the Lutín Graben (the oldest OSL age is 161.5ka, corresponding to the late Saalian). Between the end of the Saalian and late Weichselian glaciations, the Morava River abandoned the Lutín Graben for the Upper Morava Basin s.s. where it flows up to the present day. The Pleistocene fluvial style contrasts with the present-day meandering to anastomosing fluvial style of the Morava River. The Pleistocene deposits were sourced from areas corresponding to the present-day Morava River catchment including crystalline units of the eastern Bohemian Massif and the Moravo-Silesian Carboniferous Basin. They also contain a considerable input from the Bohemian Cretaceous Basin. The composition of late Weichselian deposits from the Dub nad Moravou core (34.53±3.42ka and younger) differs from the older fluvial deposits representing the other cores. It indicates that a major provenance change occurred between the latest Saalian and the late Weichselian. In the late Weichselian, the Morava River started to recycle loess deposits, which cover large areas of its catchment. Based on OSL dating, it may be assumed that the Morava River turned from degrading (between 92.6±8.03 and 34.53±3.42ka) to aggrading in style (from 34.53±3.42ka to the present day) due to coeval tectonic movements in the UMB, which are indirectly indicated by present-day seismicity, geomorphic faults and palaeoseismic evidence. Both the tectonic context and fluvial deposition styles of the Upper Morava Basin show similar features to the Upper Rhine Graben of the Alpine foreland.
•Fluvial sedimentation responded to fault tectonics in the foreland of Western Carpathians.•OSL ages of aggrading and degrading fluvial deposits indicate differential subsidence.•Fault tectonics caused switching of sediment sources in a complex bedrock geology.•Upper Morava Basin represents a small-scale analogue to Upper Rhine Graben.
The sedimentary record is affected by periodic and/or random variations in Earth's near-surface conditions, such as the Milankovitch band variations, which are responsible for small-scale cyclicity ...in distal marine settings. However, the controlling factors of normal field-scale sedimentary cycles operating on Myr-scale that dominate the stratigraphic record remain largely enigmatic. The Lower to Middle Devonian succession (Lochkovian to Givetian) of the Prague Basin, Czechia, is represented by an alternation of largely deep-marine carbonates and shales with typical duration of ~5–7 Myr. We studied the elemental geochemistry of bulk rock samples supported by XRD mineralogy and TOC concentrations. Stratigraphic distribution of the organic-productivity proxies, redox proxies, Ce/Ce* and Pr/Pr* anomalies, and TOC, supported by lithology, allochem composition and field gamma-ray spectrometry, indicates that the Prague Basin was governed by two alternating depositional modes. The oligotrophic mode is characterised by low values of productivity proxies and U/Th ratios, good bottom water oxygenation, heterotrophic skeletal producers, and facies typical of homoclinal ramp settings. The mesotrophic mode is characterised by high U/Th, elevated organic productivity, less oxic bottom conditions, relatively abundant autotrophic skeletal producers and deposition on a distally steepened ramp. The modes were in phase with the Devonian climatically driven environmental changes. We assume that the elevated silicate weathering rates during warmer periods, as previously reported, resulted in higher nutrient input to the seas setting the marine system into the mesotrophic mode. Cooler periods led to oligotrophic water conditions. The global Devonian bioevents show recurrence intervals of the same order as the trophic modes but they typically coincided with the transition between the modes. It is suggested that marine biotic assemblages and the associated carbonate production became unstable during perturbations of the carbon cycle.
•Geochemistry of sediments of the Prague Basin revealed ~Myr-scale cyclic variations.•Sedimentation was driven by alternation of oligotrophic and mesotrophic modes.•Modes were in phase with Devonian climatically driven environmental changes.•Global Devonian bioevents typically coincided with transitions between modes.
